drm/bridge: parade-ps8640: Ensure bridge is suspended in .post_disable()

The ps8640 bridge seems to expect everything to be power cycled at the
disable process, but sometimes ps8640_aux_transfer() holds the runtime
PM reference and prevents the bridge from suspend.

Prevent that by introducing a mutex lock between ps8640_aux_transfer()
and .post_disable() to make sure the bridge is really powered off.

Fixes: 826cff3f ("drm/bridge: parade-ps8640: Enable runtime power management")
Signed-off-by: Pin-yen Lin <treapking@chromium.org>
Reviewed-by: Douglas Anderson <dianders@chromium.org>
Signed-off-by: Douglas Anderson <dianders@chromium.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20240109120528.1292601-1-treapking@chromium.org

drivers/gpu/drm/bridge/parade-ps8640.c

@@ -107,6 +107,7 @@ struct ps8640 {
 	struct device_link *link;
 	bool pre_enabled;
 	bool need_post_hpd_delay;
+	struct mutex aux_lock;
 };
 
 static const struct regmap_config ps8640_regmap_config[] = {
@@ -345,6 +346,7 @@ static ssize_t ps8640_aux_transfer(struct drm_dp_aux *aux,
 	struct device *dev = &ps_bridge->page[PAGE0_DP_CNTL]->dev;
 	int ret;
 
+	mutex_lock(&ps_bridge->aux_lock);
 	pm_runtime_get_sync(dev);
 	ret = _ps8640_wait_hpd_asserted(ps_bridge, 200 * 1000);
 	if (ret) {
@@ -354,6 +356,7 @@ static ssize_t ps8640_aux_transfer(struct drm_dp_aux *aux,
 	ret = ps8640_aux_transfer_msg(aux, msg);
 	pm_runtime_mark_last_busy(dev);
 	pm_runtime_put_autosuspend(dev);
+	mutex_unlock(&ps_bridge->aux_lock);
 
 	return ret;
 }
@@ -475,7 +478,18 @@ static void ps8640_atomic_post_disable(struct drm_bridge *bridge,
 	ps_bridge->pre_enabled = false;
 
 	ps8640_bridge_vdo_control(ps_bridge, DISABLE);
+
+	/*
+	 * The bridge seems to expect everything to be power cycled at the
+	 * disable process, so grab a lock here to make sure
+	 * ps8640_aux_transfer() is not holding a runtime PM reference and
+	 * preventing the bridge from suspend.
+	 */
+	mutex_lock(&ps_bridge->aux_lock);
+
 	pm_runtime_put_sync_suspend(&ps_bridge->page[PAGE0_DP_CNTL]->dev);
+
+	mutex_unlock(&ps_bridge->aux_lock);
 }
 
 static int ps8640_bridge_attach(struct drm_bridge *bridge,
@@ -624,6 +638,8 @@ static int ps8640_probe(struct i2c_client *client)
 	if (!ps_bridge)
 		return -ENOMEM;
 
+	mutex_init(&ps_bridge->aux_lock);
+
 	ps_bridge->supplies[0].supply = "vdd12";
 	ps_bridge->supplies[1].supply = "vdd33";
 	ret = devm_regulator_bulk_get(dev, ARRAY_SIZE(ps_bridge->supplies),